Streptococcus pneumoniae (the pneumococcus) is the most common cause of severe pneumonia, and, despite the existence of licensed vaccines, annually kills half of a million children under five years old worldwide. Pneumococcal conjugate vaccines (PCVs) have the intrinsic disadvantage of a limited number of serotypes against which they provide protection, resulting in disease due to serotype replacement that reduces their impact. A protein-based vaccine could be used alone or in combination with PCVs to provide protection against all pneumococcal types in low and middle income, as well as developed countries. Antigen Discovery, Inc (ADI) of Irvine, California has developed a S. pneumoniae pan-proteome microarray, with coverage of both the core and accessory geneome, which can be used to screen antibody responses against the entire pneumococcal proteome. A proteome-scale platform for antibody immune-profiling has never before been available to the pneumococcus research community, and this technology has the power to rapidly advance our understanding of the protective immune response directed to pneumococcal proteins. The Respiratory Infections Group (RIG) at the Liverpool School of Tropical Medicine (LSTM) in the U.K. has developed a unique human model, which allows for the discovery of targets of naturally acquired and vaccine induced immunity against nasal colonization with pneumococci. In the Experimental Human Pneumococcal Carriage (EHPC) model, healthy volunteers are inoculated with a pneumococcal challenge strain, and acquisition of the challenge strain (carriage-positive) or protection from it (carriage-negative), as well as immune responses before and after challenge, are determined in mucosal secretions and peripheral blood. We will use pan-proteome microarrays to measure the anti-protein IgG and IgA levels to specific proteins before bacteria inoculation and associate these responses with protection from carriage using samples from the volunteers of EHPC trials. This tool will allow us to identify novel relevant protein targets that can be exploited as a part of a multi-component or monovalent vaccine. We expect to identify >200 immunoreactive protein targets, at least 10-20 of which are significantly associated with protection. We will attempt to prioritize 10 of the most promising candidate antigens to take forward for vaccine development.